Research Updates

Research Updates 2016

A P–T pseudo section modelling approach to understand metamorphic evolution of the Main Central Thrust Zone in the Alaknanda valley, NW Himalaya.

Abstract:-The Main Central Thrust Zone (MCTZ) in the Alaknanda valley, NW Himalaya, affected the Lesser Himalayan Crystalline Sequence and has a gradual transition to the structurally overlying Higher Himalayan Crystalline Sequence (HHCS). This boundary is defined on the basis of the following petrographic features in pelitic rocks at the base of the HHCS: (i) first appearance of microscopic needles of kyanite upon moving up-section along the transect; (ii) garnet porphyro blasts with random inclusion pattern and non-rotational growth history, which contrast with the garnet porphyro blasts in MCTZ rocks with spiral inclusion trails; and (iii) chemically homogeneous garnet porphyro blasts as opposed to the growth-zoned garnets in MCTZ rocks. Pseudosection modelling and garnet isopleths thermobarometry of pelitic rocks yield peak metamorphic conditions of 6.3–7.5 kbar and 550–582 °C in the MCTZ, and 8.0–10.0 kbar and 610–650 °C in the basal part of the HHCS. The results indicate continuity in the P–T field gradient across the contact between the MCTZ and HHCS. The MCTZ shows an inverted metamorphic sequence from biotite to garnet zones. Metamorphism in the basal part of the HHCS is in the kyanite zone, which is continuous with the inverted metamorphic sequence. Both P and T increase up-section, peak in the lower HHCS and then decrease higher up in the HHCS unit. The observations are consistent with predictions of a recently proposed thermo-mechanical model in which temperature in the shear zone rises due to viscous heating and pressure rises as a result of weakening of the rocks.

Abrupt changes in Indian Summer Monsoon strength during 33,800 to 5,500 years B.P.

Abstract:-Speleothem proxy records from north eastern (NE) India reflect seasonal changes in Indian summer monsoon strength as well as moisture source and transport paths. We have analyzed a new speleothem record from Mawmluh Cave, Meghalaya, India, in order to better understand these processes. The data show a strong wet phase 33,500–32,500 years B.P. followed by a weak/dry phase from 26,000 to 23,500 years B.P. and a very weak phase from 17,000 to 15,000 years B.P. The record suggests abrupt increase in strength during the Bølling-Allerød and early Holocene periods and pronounced weakening during the Heinrich and Younger Dryas cold events. We infer that these changes in monsoon strength are driven by changes in temperature gradients which drive changes in winds and moisture transport into northeast India.

40Ar–39Ar age constraint on deformation and brittle–ductile transitionof the Main Central Thrust and the South Tibetan Detachment zonefromDhauliganga valley, Garhwal Himalaya.

Abstract:-t40Ar–39Ar data from two sets of mylonitic two-mica granites present in the Main Central Thrust (MCT)and one leucogranite from the South Tibetan Detachment (STD) of Dhauliganga valley, Garhwal Himalaya are presented. The MCT and the STD bound the High Himalayan Crystallines (HHC) and are believed to facilitate its extrusion. Field evidence of ductile deformation in the form of tight isoclinal folding and brittle deformation in the form of back thrusts and transverse fractures are observed. The STD zone shows evidence of pervasive migration of leucogranitic melt through north dipping extensional shear zones. The ∼19.5 Ma old Malari leucogranite, present adjacent to the STD zone, experienced ductile and brittle deformation related to the tectonics of the STD. Muscovite analysis from the Malari leucogranite gives a cooling age of ∼15.2 Ma suggesting that ductile deformation in the STD zone may have ceased by∼15 Ma.40Ar–39Ar chronology of biotite from two mylonitic granites of the MCT yields cooling ages of10.8 Ma and 9.7 Ma, which we correlate with activity of the MCT at ∼10 Ma that caused rapid exhumation of the HHC.40Ar–39Ar ages of 6.4 Ma and 6.2 Ma from white mica represent newly crystallized white mica post-dating biotite cooling and indicate late stage deformation. It is inferred that, as the HHC wedge started to exhume and erode rapidly along the MCT zone at ∼10 Ma, the taper angle of the Himalayan wedge decreased to a ‘sub-critical’ stage. To regain the critical taper angle, the wedge underwent internal deformation in the form of back thrusts and duplex structures. Comparison of our data with earlier results from other sections of the MCT helps us envisage that the ∼6 Ma white mica ages can be correlated with this internal deformation event and also with the transition of deformation regime in the MCT zone from ductile to brittle.

Abstract:-The Gorkha earthquake (M 7.8) on 25 April 2015 and later aftershocks struck South Asia, killing ~9,000 and damaging a large region. Supported by a large campaign of responsive satellite data acquisitions over the earthquake disaster zone, our team undertook a satellite image survey of the earthquakes’ induced geohazards in Nepal and China and an assessment of the geomorphic, tectonic, and lithologic controls on quake-induced landslides. Timely analysis and communication aided response and recovery and informed decision makers. We mapped 4,312 co-seismic and post-seismic landslides. We also surveyed 491 glacier lakes for earthquake damage, but found only 9 landslide-impacted lakes and no visible satellite evidence of outbursts. Landslide densities correlate with slope, peak ground acceleration, surface downdrop, and specific metamorphic lithologies and large plutonic intrusions.

Abstract:Magnetotelluric(MT)measurements along a profile cutting across the Garhwal Himalaya of India are inverted to obtain 2-D electrical resistivity structures of the Himalayan wedge and of the underthrusting Indian plate. The imaged resistivity cross-section is dominated by a low-angle north-east dipping intra-crustal high conducting layer (IC-HCL)with an average thickness of 5 km. At transition from the Lesser Himalaya to the Higher Himalaya, the IC-HCL is marked by a ramp structure across which its top jumps from a depth of 8 km to 13 km. High conductivity of the layer is caused by pounding of upward propagating metamorphic fluids trapped by tectonically induced neutral buoyancy. In compression regime of the Himalaya, the mechanical weakening effects of the fluids counteract the fault-normal stresses, thereby facilitating thrust-type earthquakes on a plane imaged as the top of the IC-HCL. It is suggested that in the Himalaya collision belt, like the active subduction zone, the active seismic plane forming seat of large and great earthquakes is located a few kilometers above the top of the downgoing plate. In this tectonic setting, the high conductance ramp symbolizes a block of low shear strength and high strain, which under the deviatoric stresses release accentuated stresses into the brittle crust, thereby generating small but more frequent earthquakes in the narrow Himalayan Seismic Belt. In response to either the co-seismic pumping or the stress transfer during inter-seismic period, the upward infiltration of fluid fluxes into the over pressurized zones sufficiently reduces the shear strength of local thrusts and shear zones, turning these into locales of concentrated seismicity.

Abstract:-A simple, single sample preparation involving pressed rock powder pellets was utilized to determine the trace and ultra-trace abundances of petrogenetically important elements including high field-strength elements and REEs by laser ablation-ICP-MS. One of the elements predetermined by XRF spectrometry served as an internal standard. The influence of sample preparation parameters grain size, pellet compactness and amount of binding media) on analytical performance was also investigated, including sample homogeneity issues at the laser sampling scale. Line scanning with a high repetition frequency (20 Hz) and large beam diameter (200 lm)ensured ablation from a larger sample surface area,eliminating issues related to sample heterogeneity.A median grain size of about 10 lm for silicate rock powders was found to be sufficiently representative at this scale of laser sampling. Granitic rocks or samples containing resistant minerals such as zircon needed extragrinding to achieve grain sizes down to < 5 lm for better precision for elements that are concentrated in these phases. Using 137Ba as an internal standard, reasonable accuracies within 15–20% for most of the high mass trace elements were achieved; in the case of low mass elements, it may deviate up to 40%. Precision of measurements rarely exceeded 15% RSD.